builder/common: add common StepConnectSSH for builders
builder/amazonebs: switch to common SSH connect step
builder/common: if cancel during SSH, cancel the attempts
builder/digitalocean: use common connect ssh
builder/virtualbox: use common SSH connect step
builder/vmware: convert to cmmon ssh step
CHANGELOG
Merge branch 'common-ssh'

This unifies all the builders so that they all wait for SSH using a common
step available in builder/common. This common step has the best practices of
every previously unique step, such as retrying SSH handshakes, limiting
the number of SSH handshakes, timeouts, and so on.